Anatomy & Scaling

Basal metabolic rate follows Kleiber’s 3/4-power law across mammals:

\[ BMR \;=\; 3.4\,M^{0.75}\ \text{(W, kg)} \]

A 2 300 kg white rhino therefore burns ~1 100 W basally — 16× a 70 kg human. Food intake scales near-proportionally with BMR: white rhinos graze ~55 kg dry-matter grass per day, black rhinos ~20–25 kg of browse. Population energetic calculations (Owen-Smith 1988) predict that African savannas need >2 kg dry forage ha^-1 day^-1 to support stable rhino densities.

Rhino limbs are graviportal: straight, columnar bones with minimal muscle mass above the elbow and knee. This architecture minimises bending-moment loads at the cost of cursorial speed. Even so, white rhinos charge at ~45 km h^-1and black rhinos at ~55 km h^-1 (Alexander 1989, Owen-Smith 1988). Footprint area per unit body mass is ~2 cm² kg^-1, producing plantar pressures of ~400 kPa — a key constraint on substrate choice (M3, charge mechanics).

Bone allometry: cross-sectional area scales with M^2/3 while required support stress scales with M, so safety factors decline with body mass — the Galileo problem that caps terrestrial vertebrate size.

Mammalian heart mass is a constant ~0.6% of body mass across 8 orders of magnitude. Heart rate scales as HR ∝ M^-0.25 (Stahl 1967), with constant cardiac output per beat normalised to body mass. For the white rhino:

\[ m_{heart} \approx 0.006\,M \approx 14\ \text{kg},\qquad HR \approx 241\,M^{-0.25}\approx 35\ \text{bpm} \]

Resting cardiac output is ~80 L min^-1; stroke volume ~2 L. Blood volume scales with body mass: ~150 L in a 2.5 t rhino. Exercise HR peaks near 100 bpm during charges (Allen 2006 field-veterinary telemetry).

Rhinos are hindgut fermenters like horses: fibre is digested in an enlarged caecum and colon by bacterial and protozoan microbiota. Passage time is fast (~45 h) compared with foregut-ruminant giraffes or cattle (~75 h), so rhinos can process larger volumes of low-quality forage but extract less protein per gram than ruminants. This trade-off selects for bulk grazing (white rhino) or selective browsing (black rhino, Sumatran).

Volatile fatty acids (acetate, propionate, butyrate) from microbial fermentation provide 30–50% of total metabolic energy. Microbiome sequencing (Gibson 2019) shows rhino caecal flora is dominated by Firmicutes and Bacteroidetes with species-specific differences (white vs black) matching the grazer/browser diet divergence.

Rhino brains weigh 500–700 g (white rhino) — modest in absolute terms and EQ (encephalisation quotient) ~0.5 relative to the mammalian regression. This places rhinos below elephants (EQ 1.5–2.0) and cetaceans but above most ungulates. Behavioural repertoire reflects this: rhinos have excellent olfactory and auditory cognition, good social memory over years, but limited problem-solving tested in captivity.